This invention relates to a method and a system for the production of hydrocarbons and hydrocarbon compounds which includes the use of a Fischer-Tropsch synthesis reactor and process, utilizing an iron-based catalyst, in combination with processes for converting hydrocarbon-containing gases in general, and in particular gases rich in methane, into hydrogen and carbon monoxide from such gases.
Considerable research and development work has been undertaken in the past to commercially apply the Fischer-Tropsch synthesis of hydrocarbons, from a wide variety of carbonaceous and hydrocarbon starting materials.
A compendium of some of the prior work with Fischer-Tropsch synthesis technology is contained in the Bureau of Mines Bulletin 544 (1955) entitled Bibliography of the Fischer-Tropsch Synthesis and Related Processes by H. C. Anderson, J. L. Wiley and A. Newell.
The product distribution and yields from specific Fisher-Tropsch reactions with iron catalysts have also been examined by Charles N. Satterfield and George A. Huff, Jr. in an article entitled Carbon Number Distribution of Fischer-Tropsch Products Formed on an Iron Catalyst in a Slurry Reactor, Journal of Catalysis 73, 187-197 (1982), wherein the Shultz-Flory distribution is examined with respect to various catalyst systems.
In addition, the article entitled Fischer-Tropsch Processes Investigated at the Pittsburgh Energy Technology Center Since 1944 by Baird, Schehl, and Haynes in Industrial and Engineering Chemistry, Product Research and Development, 1980, 19, pages 175-191, describes various Fischer-Tropsch reactor configurations.
The foregoing articles describe in considerable detail how specific catalysts can be employed in various reaction vessel configurations under conditions which favor the conversion of carbon monoxide and hydrogen into specific product groups.
There have only been a few instances wherein the Fischer-Tropsch reaction has been incorporated into a complete system, starting with a solid or gaseous feed stock. Germany placed several plants in operation during the 1930's and 1940's using coal as the feed stock, referenced in Twenty-Five Years of Synthesis of Gasoline by Catalytic Conversion of Carbon Monoxide and Hydrogen, Helmut Pichler, Advances in Catalysis, 1952, Vol. 4, pp. 272-341. In addition to the foregoing, South Africa has been using Fischer-Tropsch technology based upon this German work for the past 35 years to produce gasoline and a variety of other products from coal, referenced in Sasol Upgrades Synfuels with Refining Technology, J. S. Swart, G. J. Czajkowski, and R. E. Conser, Oil & Gas Journal, Aug. 31, 1991, TECHNOLOGY. There was also a Fischer-Tropsch plant built in the late 1940's to convert natural gas to gasoline and diesel fuel described in Carthage Hydrocol Project by G. Weber, Oil Gas Journal, Vol. 47, No. 47, 1949, pp. 248- 250. These early efforts confirmed that commercial application of the Fischer-Tropsch process for the synthesis of hydrocarbons from a hydrocarbon-containing feed stock gas requires solving, in an economical manner, a set of complex problems associated with the complete system. For example, initially, it is important for the hydrocarbon-containing feed stock to be converted into a mixture consisting essentially of hydrogen and carbon monoxide before introduction of the mixture into the Fischer-Tropsch reactor. Economic operation of specific sizes of Fischer-Tropsch reactors, generally requires the ratio of hydrogen to carbon monoxide to be within well established ranges. The Hydrocol plant, referenced hereinbefore, used partial oxidation of natural gas to achieve a hydrogen to carbon monoxide ratio of about 2.0. An alternative approach to partial oxidation uses steam reforming for converting light hydrocarbon-containing gases into a mixture of hydrogen and carbon monoxide. In this latter case, steam and carbon dioxide, methane and water are employed as feed stocks and carbon dioxide can be recycled from the output of the reformer back to its inlet for the purpose of reducing the resultant hydrogen to carbon monoxide ratio.
There are therefore two primary methods for producing synthesis gas from methane: steam reforming and partial oxidation.
Steam reforming of methane takes place according to the following reaction: EQU (1) H.sub.2 O+CH.sub.4 .revreaction.3H.sub.2 +CO
Since both steam and carbon monoxide are present, the water gas shift reaction also takes place: EQU (2) H.sub.2 O+CO.revreaction.H.sub.2 +CO.sub.2
Both of these reactions are reversible, i.e., the extent to which they proceed as written depends upon the conditions of temperature and pressure employed. High temperature and low pressure favor the production of synthesis gas.
Partial oxidation reactions utilize a limited amount of oxygen with hydrocarbon-containing gases, such as methane, to produce hydrogen and carbon monoxide, as shown in equation (3), instead of water and carbon dioxide in the case of complete oxidation. EQU (3) 1/2O.sub.2 +CH.sub.4 .fwdarw.2H.sub.2 +CO
In actuality, this reaction is difficult to carry out as written. There will always be some production of water and carbon dioxide; therefore the water gas shift reaction (2) will also take place. As in the steam reforming case, relatively high temperatures and relatively low pressures favor production of synthesis gas.
The primary advantage of partial oxidation over steam reforming is that once the reactants have been preheated, the reaction is self-sustaining without the need for the addition of heat.
Another advantage of partial oxidation is the lower ratios of hydrogen to carbon monoxide normally produced in the synthesis gas which ratios better matches the desired ratio for use in the Fischer-Tropsch synthesis of hydrocarbon liquids in the overall process.
A still further advantage of partial oxidation resides in the elimination of a need for the removal of carbon dioxide and/or hydrogen from the synthesis gas before being fed to the synthesis reactors.
Adjustment of the hydrogen to carbon monoxide ratio can be achieved by removal of excess hydrogen using, for example, a membrane separator. However, this approach requires additional capital equipment and can result in lower oil or liquid hydrocarbon yields due to a loss of hydrogen to the process.
In order for the overall process considerations to be used in a manner which can produce economical results whether employing either steam reforming or partial oxidation of a feed stock, the Fischer-Tropsch reactor must typically be able to convert at least 90% of the incoming carbon monoxide. If a 90% conversion efficiency is to be achieved in single pass operation and hydrogen is not removed before introduction of the gas stream into the reactor, the build up of hydrogen due to the excess of hydrogen will necessitate a larger reaction vessel to maintain a sufficiently long residence time in the reaction vessel. Recycle of unreacted hydrogen and carbon monoxide from the outlet of the Fischer-Tropsch reactor back to its inlet is commonly employed to achieve the required conversion. However, when an excess of hydrogen is employed, an even greater excess of unreacted hydrogen will build up under such a recycle operation. This condition, in turn, can necessitate an even larger reaction vessel or alternatively the hydrogen removal described must be employed.
Major drawbacks to the commercialization of many of the prior processes were the high cost of product specific catalysts, and an unacceptable overall process conversion efficiency of the carbon input into liquid hydrocarbon output, particularly, when an inexpensive catalyst was utilized.
The two catalyst types attracting the most serious attention for the Fischer-Tropsch reaction are either cobalt based or iron-based catalysts. In practice, a cobalt-based catalyst will favor the following reaction: EQU (4) CO+2H.sub.2 .fwdarw.(--CH.sub.2 --)+H.sub.2 O
While an iron catalyst will favor the following overall reaction (due to its high water gas shift activity): EQU (5) 2CO+H.sub.2 .fwdarw.(--CH.sub.2 --)+CO.sub.2
Theoretically, cobalt-based catalysts can produce higher conversion yields than iron-based catalysts since cobalt can approach 100% carbon conversion efficiency, whereas iron tends toward 50% carbon conversion efficiency during the Fischer-Tropsch synthesis reaction since the reaction (5) favors the production of carbon in the form of CO.sub.2. The major drawbacks encountered are, first, that cobalt-based catalysts are very expensive compared to iron-based catalysts and, further, if the Fischer-Tropsch technology were embraced on a large scale worldwide, the higher demand for relatively scarce cobalt might drive the cost even higher.
The use of cobalt-based catalysts have typically included recycle of tail effluent back to the inlet of the Fischer-Tropsch reactor to achieve 90% conversion primarily because cobalt favors formation of water. Water is well known to be a strong inhibitor of either catalytic reaction schemes. Thus, as the reaction proceeds in the presence of water, not only is the concentration of reactants less, but the concentration of inhibiting water vapor is greater. In practice, generally 70% carbon monoxide conversion is the maximum attainable in single-pass operation using a cobalt-based catalyst. Iron-based catalysts, which favor carbon dioxide formation permit up to 90% of the theoretical conversion of carbon monoxide per pass without great difficulty, and without the formation of additional water, thereby eliminating the necessity for effluent recycle back to the inlet of the Fischer-Tropsch reactor.
It has generally been considered undesirable to form CO.sub.2 in the Fischer-Tropsch synthesis reaction as happens using iron-based catalysts and therefore many process schemes use cobalt-based catalysts including the recycle of some of the reactor effluent directly back into the Fischer-Tropsch reactor. In summary, therefore, iron-based catalysts, while efficient in converting carbon monoxide into the products shown in equation (2), have previously been limited in overall carbon conversion efficiency since their use favors the production of carbon dioxide, and therefore, they were not as efficient in overall carbon conversion efficiency as the process schemes utilizing cobalt based catalysts.
The Fischer-Tropsch synthesis has therefore been used in combination with an up-stream steam reforming reactor which must then be followed by CO.sub.2 removal from the carbon monoxide and hydrogen reaction products before the CO and H.sub.2 produced by the steam reforming reaction are subjected to a Fischer-Tropsch reaction using cobalt-based catalysts.
In selecting a suitable catalyst for use in a system which favors reaction (5), several considerations are important. In the Fischer-Tropsch synthesis using appropriately designed equipment, the hydrogen to carbon monoxide feed ratio to the Fischer-Tropsch reactor will optimally be in the range of from 1.0 to 2.0 parts of hydrogen for every part of carbon monoxide. This is necessary in order to obtain reasonably acceptable percent conversion of carbon monoxide into hydrocarbon per pass through the Fischer-Tropsch reactor without the undesirable formation of carbon in the catalyst bed.
In order to adjust the H.sub.2 /CO ratio into the range of optimum ratios described hereinbefore for the catalyst selected, it is necessary and typical that an additional stage of hydrogen removal, by a membrane or the like, is inserted into the product stream between the steam reformer and the Fischer-Tropsch reactor. This hydrogen removal is necessary to produce the proper ratio of hydrogen to carbon monoxide in the steam reforming reaction product stream entering the Fischer-Tropsch reactor.
The present inventors have discovered that notwithstanding the foregoing difficulties, economic viability for a natural gas to oil conversion process using steam reforming or partial oxidation and a Fischer-Tropsch synthesis using an iron-based catalyst can be achieved. The present invention includes a solution to the problems of reducing the formation of excess hydrogen from the reformer or partial oxidation unit and increasing the overall carbon conversion efficiency for the entire carbon input to the system when using specifically prepared iron catalysts. As will be shown hereinafter, the carbon dioxide produced by such iron catalysts, which production contributes to the low carbon conversion efficiencies previously discussed, can be used to solve both the excess hydrogen and low overall carbon conversion efficiency problems.